Prostate cancer is the most common cancer of men in the United States and represents their second leading cause of cancer death. Localized prostate cancer typically is treated with surgery and/or radiation, and recurrent disease can be controlled temporarily with androgen ablation. However, almost all prostate carcinomas eventually become hormone refractory and then rapidly progress. Prostate cancer has proven to be largely resistant to conventional chemotherapy, and currently there is no effective treatment for advanced, metastatic disease. Thus, there is an urgent need for novel, molecularly targeted therapies that can selectively kill prostate cancer cells without systemic toxicity. Prostate-specific membrane antigen (PSMA) is a well-characterized glycoprotein whose expression is largely restricted to prostate epithelial cells. In normal tissues, PSMA exists as a splice variant that lacks the transmembrane domain and is thereby retained in the cytoplasm. On tumor cells, however, PSMA is expressed as a type 2-membrane protein with a large extracellular domain. PSMA expression increases approximately 1000-fold with disease progression, and the protein is rapidly internalized both constitutively and following antibody binding. For these reasons, PSMA is widely regarded as being an attractive target for antibody-based therapy of prostate cancer. We seek to develop novel and highly potent PSMA-targeted immunotoxin therapies for prostate cancer. We previously developed a novel panel of fully human monodevelopment of a therapy that augments efficacy of chemotherapy would be a major advance. Clonal antibodies (mAbs) that bind with high affinity to epitopes scattered throughout the extracellular domain of PSMA. In this project, we will utilize these mAbs to specifically deliver ultrapotent cytotoxic agents to PSMA-expressing tumor cells in vitro and in vivo. The project proposes the first head-to-head evaluation of immunotoxins based on potent protein and drug cytotoxins. We will first optimize procedures for conjugating the protein and drug toxins to our most promising fully human PSMA mAbs. The protein and drug immunotoxins then will be comparatively evaluated for potency and selectivity in eliminating prostate cancer cells in vitro. The optimized immunotoxins then will be tested for immunogenicity, pharmacology, and antitumor effects in the best available preclinical models of human prostate cancer. These studies will enable us to identify the mAb-toxin construct that has the overall greatest therapeutic potential. This product candidate then will be advanced into human clinical testing in the Phase II project. The overall goal of this project is to optimally exploit PSMA as a molecular target for cancer therapy using innovative technologies in the areas of fully human antibodies, ultrapotent cytotoxins, and small-animal models of human prostate cancer.